TP53 and cancer: Wee1 is a tyrosine kinase that regulates the G2/M cell cycle checkpoint by phosphorylating and inactivating CDK1 in response to extrinsic DNA damage and errors in DNA synthesis, thereby preventing mitotic entry.325 Cancer cells often have a deficient G1/S checkpoint which frequently via mutation of p53 to leave them reliant on the G2/M checkpoint to avoid mitotic catastrophe.